1. Field of the Invention
The present invention relates to new isocyanurates having isocyanate groups, based on mixtures of hexamethylene diisocyanate and 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl-cyclohexane (isophorone diisocyanate or IPDI), to a process for the production thereof by the catalytic trimerization of a mixture of the above-mentioned diisocyanates and to the use thereof as isocyanate component in polyurethane lacquers.
2. Description of the Prior Art
The preparation of isocyanurates having isocyanate groups by the catalytic trimerization of some of the isocyanate groups of organic diisocyanates is known from numerous publications (see, for example, British Pat. Nos. 809,809 or 856,372; German Pat. No. 1,201,992, German Offenlegungsschrift Nos. 1,644,809; 1,670,667; 2,616,415; 2,616,416; 2,644,684; 2,724,914; 2,726,749; and 2,806,731; U.S. Pat. Nos. 2,801,244 and 3,394,111 or Published European Application No. 10589).
The isocyanurates having isocyanate groups prepared according to these prior art processes are valuable starting materials, in particular for the production of two-component polyurethane lacquers, in which they may be advantageously used as isocyanate component. The isocyanato-isocyanurates having aromatically-bound isocyanurate groups are particularly suitable for the production of lacquers of great hardness and elasticity, while the isocyanato-isocyanurates having aliphatically- or cycloaliphatically-bound isocyanate groups are particularly used for the production of weatherproof, light-fast lacquers. The isocyanato-isocyanurates described in the above-mentioned publications having aliphatically- or cycloaliphatically-bound isocyanate groups still suffer, however, from the disadvantage that they cannot simultaneously meet all of the practical requirements. For example, the isocyanato-isocyanurates based on aliphatic diisocyanates, such as hexamethylene diisocyanate, are not always suitable for uses which are steadily increasing in scope and significance, which uses require a good solubility in slightly polar, physiologically harmless solvents (e.g. petroleum fractions), because for the preparation of the solutions, more polar solvents must be simultaneously used or because the dilutability of the solutions is restricted by the above-mentioned non-polar solvents.
Moreover, these isocyanato-isocyanurates are usually liquid (even in blocked form) and for this reason, are unsuitable for use in powder lacquers. In addition thereto, lacquers which are produced using these polyisocyanates usually exhibit a low initial hardness which impairs the rapid further processing thereof.
Isocyanato-isocyanurates which are obtained by trimerizing cycloaliphatic diisocyanates do indeed exhibit good solubilities in slightly polar solvents, but in turn require considerable quantities of these solvents for the preparation of solutions of conventional processing viscosities. This fact considerably restricts the usefulness thereof in low solvent systems.
The products do have softening points which are clearly above room temperature. However, they impart to the resulting coatings a certain brittleness and a restricted elasticity which limits the use thereof particularly in lacquers, on which are imposed exacting requirements with respect to elasticity even at low temperatures.
Surprisingly, it has now been found that the isocyanato-isocyanurates according to the present invention and described in detail below, based on hexamethylene diisocyanate and IPDI, compare advantageously with the isocyanato-isocyanurates described in the above-mentioned publications by virtue of the favorable property spectrum thereof. The compounds or mixtures according to the present invention may be dissolved into clear, highly concentrated, low viscosity solutions in slightly polar solvents which may be further diluted as required using the same solvents without cloudiness occurring. The lacquers produced using the compounds or mixtures according to the present invention are also distinguished by a desirable hardness and outstanding elasticity of the resulting lacquer even at low temperatures.
This observation is surprising, although there is also the passing reference in some of the above references, for example in British Pat. No. 809,809 or in German Offenlegungsschrift Nos. 2,644,684; 2,724,914 or 2,726,749, after enumerating long lists of suitable aliphatic or cycloaliphatic diisocyanates, that mixtures of the isocyanates mentioned may also be used. However, from this reference alone, the skilled man could not infer any suggestion whatsoever of selecting hexamethylene diisocyanate and IPDI from this list in order to prepare mixed trimers from these two diisocyanates. In fact, there is no concrete indication in the above-mentioned publications of choosing these two-diisocyanates.